The terminology, hydraulic shock, covers a number of physical phenomena, in the case of which water produces a strong mechanical shock. The cause rests in the small compressibility of water. In the case of systems of vapor in the form of steam, hydraulic shocks occur in the form of condensation shocks, which are observable, among others, in the case of pressure increases. In principle, three phenomena are distinguished in this regard, which all fall under the label, condensation induced hydraulic shock. One speaks of steam hammer, when steam bubbles implode in water. Occasionally, this principle is referred to in the literature also as hydraulic shock. A droplet impact, in contrast, is present, when droplets impact with high velocity on a surface. And, finally, cavitation refers to a microscopic form of steam hammer.
Condensation induced hydraulic shocks are of importance in vapor systems, pipes containing a condensing gas, especially vapor lines, however, also boilers or thermal solar plants. For example, they can occur in the case of heating a fluid by targeted introduction of vapor into a liquid. However, also during operation, unwanted condensation induced hydraulic shock can occur, when condensate is insufficiently removed from the respective media-containing component such as a container, boiler or pipe, and hot vapor flows into the cold liquid. As a result, pressure spikes of up to a number of hundred bar can arise with the possible result of severe damage to the respective component. Condensation induced hydraulic shock belongs to the most frequent causes of significant accidents, for example, in steam power plants or steam boiler plants.
The underlying mechanisms are known from a large number of publications. In the following, by way of example, the occurrence of steam hammers will be described in greater detail. When a steam bubble is entrained within a very much colder condensate, the steam bubble is cut off, e.g insulated, from further steam-, respectively energy, supply. As a result, the steam bubble transfers its energy to the condensate and cools down to the temperature of the condensate. Correspondingly, also the pressure in the steam bubble falls from a starting value of some bar to a few mbar. Due to the high heat transfer coefficient between steam and condensate, this chain of events occurs in a very short time interval, usually within a few milliseconds.
During this chain of events, the condensate surface, which surrounds the steam bubble, collapses toward the center of the steam bubble. In the center, the surfaces of the condensate coming from the different spatial directions impact on one another. Correspondingly, high pressure spikes occur, which lead spontaneously, however, also as a result of repeated occurrences, to significant damage to the respective media-containing component as well as to measuring apparatuses possibly mounted therein, respectively thereto, until a bursting occurs with resultant escape of the respective medium into the environment.
The conditions for the occurrence of steam hammers in a pipe flowed through by a fluid are listed, for example, in the Proceedings of the 8th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety in the paper by I. F. Barns, L. Varga and Gy. Èzsöl entitled “Steam Condensation Induced Water Hammer Simulation for Different Pipelines in Nuclear Reactor” (http://www.kfki.hu/˜barnai/N8P0220.pdf, downloaded on Aug. 1, 2014):                a) The pipeline must be horizontal.        b) The temperature difference between steam and condensate must be at least 20K.        c) The ratio of length to diameter of the pipeline must be greater than 24.        d) The Froude number, which is the ratio of inertial forces to gravitational forces within a hydrodynamic system, must be less than 1.        e) The neighboring steam volume must be sufficiently large.        f) The occurring pressure spikes must be at least 10 bar over the maximum allowable operating pressure, in order to bring about significant damage.        
The list makes clear that the occurrence of condensation induced hydraulic shock, especially water hammer, depends, as a rule, on a number of factors acting together. The prediction of when a condensation induced hydraulic shock, especially a steam hammer, will occur is, correspondingly, in no way trivial. Since, however, the damage caused thereby can be immense, an early warning system for the occurrence of condensation induced hydraulic shock would be desirable.